Earlier on this year we had a partial solar eclipse that was viewable from the UK. Luckily it was cloudy as I couldn't find any of those cardboard glasses or figure out how to safely photograph the eclipse. This did, however, start me on a journey to try and figure out a cheap and easy way to safely view solar events. I remember a long time back there was a transit of Venus, which was cool also.
Searching on the web revealed that you could buy a nifty folding viewer with a lens and a mirror that projected the image onto a white sheet for safe viewing. The cardboard version was £45 which I thought was ridiculous so I set out to make my own. I started researching and pretty soon realised that a pin-hole was the ideal lens for this job. It's simple to make and cuts out an awful lot of light!
This is a bit of a free-form instructable as I'm still experimenting and changing things around to get things better. But the bottom line is that I can totally safely and fairly comfortably observe the sun, so my main objective has been met. I think so far I've spent less than £5 on materials, and most of that was in the "I'm sure I've got one somewhere but can't find it right now" category!
Step 1: Gather Materials
Here are the tools and other materials I used. Hammer, nail, emery paper and drinks can all went to make a pinhole. Not shown here is a small mirror. I couldn't find one, so bought one from ebay. It was a folding camping mirror and very cheap. The idea of using a mirror is to increase the length of the system without making it bigger, and also to bring the image to somewhere easier to view. The cardboard boxes, gaffer tape and bulldog clips are used to make a housing for the whole thing. I was lucky to have a supply of identical sized boxes available. Also, after I took these photos I found a length of black material that acted as a hood.
Step 2: Making the Pinhole
The ideal pinhole apparently is a perfectly round hole is a very thin but still stiff piece of material. According to the internet, drinks can aluminium is ideal, so I cut a rectangle out and sanded it down until I got bored, hopefully making it thinner. Then I hammered a needle through it before doing some more sanding to remove the pushed out bit that you get from hammering a nail through.
According to wikipedia the ideal diameter d = 1.9 x square root of (f x lambda) Lambda is the wavelength of light, around 550 nanometers and my focal length was one meter. (The boxes gave a distance of about 50cm and the mirror allowed me to double it.) This made my ideal diameter 1.4mm. I then thoroughly confused the staff in a local sewing shop and came away with a couple of stout embroidery needles.
Step 3: Mounting the Pinhole
My original plan was to have two boxes at right-angles and have the pinhole at the top of the upright box, in the corner. Then, with a mirror in the lower box, I could reflect the image and view it next to the pinhole on the other side. However I never made this work and changed my plan. Anyway, here are the pictures of me marking and cutting the hole for the pinhole, and then sealing the entire box to prevent stray light from entering. Finally I created the Dementor Box by gaffer taping a piece of black cloth to it to make a viewing hood.
Step 4: Setting It Up - Plan B
I finally settled on a layout that was popular in Victorian times. The pinhole lens faces the sun, and a mirror is mounted at 45 degrees, reflecting the image upwards onto a screen. In my case the screen is a scrappy bit of greaseproof paper. If you had a bit more time and few less children running around the garden, you'd make this neater and tidier. But the bottom line is that it worked. In that last photo you can see a circle, and that is the sun. As it set behind some trees, I could clearly see the branches across the disc of the sun, so for eclipses and transits it should be perfect!
Step 5: Setting It Up - Plan A
It was another sunny day today so I set up the original plan A - as seen in the first photo. Simply by cutting a couple of slots into the top box with the pinhole, I made a hinge that could angle the lens. Instead of greaseproof paper, I used a sheet of printer paper as a screen. I tacked it inside the top box, to the side of the pinhole. This time I set up the mirror by leaning down so my eye was about level with the pinhole with the sun behind me and then changing it til I could see myself. Then I slid the top box into place and moved it to and fro until I saw the image and then clipped it into position. There's some weird refraction or interference pattern thing going on as there's two suns but the image is lovely and clear. The best thing is that it works without the black cloth hood so if there was a solar event we could all sit around and watch it.